The components of a naturally-occurring genetic recombination between viral and host DNA have been defined and purified. One of the proteins required for this reaction, the bacteriophage lambda Int protein, has been shown to behave as a topoisomerase, i. e. to break and reseal the strands of DNA so as to relax supertwisted DNA. This is the first case in which a topoisomerase has been directly implicated in genetic recombination and provides new insight into the function of these ubiquitous proteins. Int protein also binds specifically to DNA, covering two closely spaced sequences one of which is centered at the site of the genetic recombination. Other studies of site specific recombination have uncovered unusual conditions that permit relaxed DNA to function as the substrate for this reaction, clearing the way for comparative studies to determine the role of DNA supercoiling in a biological reaction.